This invention relates to a nuclear power plant having a containment structure enclosing a nuclear reactor vessel and more particularly to a power plant having a containment structure which can be passively cooled should a postulated design basis event occur.
Commercial nuclear power plants generally have reactor vessels, steam generators and auxiliary equipment enclosed within single or double containment structures designed to contain radioactive materials in the atmosphere surrounding the vessels, piping and auxiliary equipment when the plants are operational or in the course of postulated design basis events such as earthquake, equipment failure, electrical interruption and the like. Single containment structures generally have cylindrical sidewalls with top and bottom domed ends. Double containment structures generally have two generally concentric, spaced apart sidewalls and domed ends. Generally speaking, the pressure in the annulus between the concentric containment walls is maintained slightly below atmospheric pressure to minimize leakage from within the containment structure to the environment.
Conventional nuclear power plants also have containment cooling systems designed to transfer heat from within the containment structures in the course of operation or in the course of postulated design basis events. Thus, for example, in-containment air conditioning units or circulating water systems including in-containment piping arrangements or heat exchangers may be utilized to transfer heat from the atmosphere within the containment structures to ultimate heat sinks such as the general plant atmosphere and nearby rivers via cooling towers or other types of heat exchange devices located outside of the containment structure. These plants have active components such as fans and pumps for circulating air in the containment structures and water through the heat exchangers in order to drive the heat transfer across the heat transfer surfaces of the cooling systems. However, in the event of equipment or electrical failure, these cooling systems may not be sufficient to transfer sufficient heat from the containment structure in all postulated design basis events in order to control the temperature and pressure of the containment atmosphere.
Future nuclear plants will include "passive" systems for safely continuing to operate for an extended period of time to shutdown a plant without electrical power or human intervention after the postulated occurrence of a design basis event. U.S. Pat. Nos. 4,753,771 and 5,049,353, both to Conway et al. and assigned to the assignee of the present invention, disclose a passive containment cooling system which will function in nuclear power plants having pressurized water reactors or boiling water reactors without active components. The cooling system utilizes naturally induced air flow within the containment vessel, thermal conduction through a containment shell and gravity flow of water over the containment shell to passively transfer heat generated in the reactor vessel and other process equipment to the general atmosphere surrounding the containment structure. Such passive systems and other passive safety grade systems are designed to function for relatively long periods of time up to about seventy-two hours or more without the need for human actions, AC power or heating and air conditioning in the event of the occurrence of a postulated design basis accident.
The passive cooling systems of U.S. Pat. Nos. 4,753,771 and 5,049,353 are designed to be used in connection with single containment structures to cool the atmosphere within the containment and to condense steam which may be released into the containment atmosphere. Such passive cooling systems (which depend upon thermal conduction through the containment shell) are not likely to be utilized in connection with plants having double containment structures because the inherent resistance to heat transfer through concentric shells and an intermediate air space severely reduces heat transfer through the containment walls. In addition to the resistance to thermal conduction presented by two spaced walls, thermal convection of the air in the annular space between the spaced walls of double containment designs will be reduced because the annular space will be sealed and the air filtered.